1. Field of the Invention
This invention relates generally to memory systems and more particularly to a system and method to compensate for data defects within a magneto-optical computer memory device.
2. Description of the Background Art
Providing reliable storage and retrieval techniques for digital information is an important consideration of manufacturers, designers and users of computing systems. In magneto-optical storage devices that use flying heads, digital data is written onto and read from the front surfaces of rotating disks of MO storage media. Referring now to FIG. 1(a), a plan view of a front surface 112 of a magneto-optical storage medium 110 is shown. In magneto-optical storage devices, digital data is typically written into and read from a series of concentric or spiral tracks 114 located on the surface 112 of storage medium 110. In practice, the digital data is read from the front surface 112 of storage medium 110 by projecting a laser-generated light beam from a flying head onto a selected track 114 while storage medium 110 is rotating, and then sensing the amplitude and polarization of light reflected back from the surface 112 of storage medium 110.
Referring now to FIG. 1(b), a cross-sectional view of the FIG. 1 magneto-optical storage medium 110 is shown. In operation, a flying head (not shown) is positioned above front surface 112. FIG. 1(b) includes several examples which illustrate possible causes of unreliable or invalid data in magneto-optical storage devices. The FIG. 1(b) examples include a corrosion defect 116, particulate contamination 118 and a "bright spot" 120. These examples are presented for purposes of illustration and defective data may readily be caused by various other factors.
The FIG. 1(b) examples each significantly alter the data read from the surface 112 of storage medium 110. Corrosion defect 116 and particulate contamination 118 each reduce the reflective properties of surface 112. This change in reflectivity reduces the MO signal amplitude of data read from storage medium 110. In contrast, bright spot 120 causes increased reflectivity in surface 112. This increase also reduces the mark sizes of data stored on storage medium 110, because bright spot 120 reflects the laser beam used to heat storage medium 110 during the data writing process. Bright spot 120 thus prevents data from being effectively written to storage medium 110.
In addition, front surface media is more prone to significant data defects due to causes such as particulate contamination 118. Conventional MO media has an active layer buried some distance below the media surface. Particulate contamination on conventional MO media may therefore be out-of-focus and hence unreadable. In contrast, the present invention uses front surface media 110 above which a flying recording head (containing optics and magnetic-field modulation coils) is used to record and sense MO data marks directly from the front surface 112 of storage medium 110. Particulate contamination 118 on front surface 112 thus has a greater impact on the data signal read from storage medium 110.
As discussed above, corrosion defects 116, particulate contamination 118, and bright spots 120 may cause data defects in front surface magneto-optical storage devices. Furthermore, magneto-optical storage devices may be unable to compensate for these data defects. Magneto-optical devices often contain automatic gain controls (AGCs) to control data amplitude and phase-locked oscillators (PLOs) to synchronize the data flow. A significant dropout or data defect, however, may disrupt AGC and PLO operation so severely that the magneto-optical device is unable to restore normal data amplitude or data synchronization. From the above discussion,. it becomes apparent that the magneto-optical data is not sufficiently robust to defects. Therefore, an improved system and method are needed to compensate for data defects within a front surface media magneto-optical memory device.